Cellulose consists of a linear chain of β1-4 linked D-glucose residues having a molecular structure as shown in FIG. 1. This long linear glucose chains are tightly bundled together in microfibrils and are non-covalently linked together by hemicelluloses (Kolpak F J, Blackwell J. Determination of the structure of cellulose II. Macromolecules 1976; 9:273-278; Carpita N C, Gibeaut D M. Structural models of primary cell walls in flowering plants: consistency of molecular structure with the physical properties of the walls during growth. Plant J 1993; 3:1-30). Cellulose forms crystalline, insoluble microfibrils in plant cell walls which are recalcitrant to enzymatic hydrolysis. This recalcitrance is the bottle-neck in cellulosic ethanol production (Himmel M, Ding S, Johnson D, Adney W, Nimlos M, Brady J W, Foust T D. Biomass recalcitrance: Engineering plants and enzymes for biofuels production. Science 2007; 315: 804). On the other hand hemicelluloses are the most complex group of non-starch polysaccharides and it consists of polymer of xylose, arabinose, galactose, mannose which are often highly branched and connected to other cell wall structure.
Cellulose, having been the most abundant biological material in the world, is a vast, renewable resource that could help meet the world's energy needs. But the production of fermentable sugars from biomass by using of cellulolytic enzymes is not yet able to compete economically due to the inefficiency of the currently used cellulolytic enzymes. There is a need for research aimed at increase efficacy of celluloytic enzymes that can be used to generate fermentable sugar from lignocellulosic materials with reduced cost. For complete digestion of cellulose to glucose the cellulase systems requires three classes of enzymes, β-1,4-endoglucanases (EGL), exoglucanases/cellobiohydrolases (CBH), and β-glucosidase (BGL). During hydrolysis process, endoglucanase first randomly cleaves different regions of crystalline cellulose, producing chain ends. Cellobiohydrolases then sequentially release cellobiose from the end of the cellulose polymer. Finally, β-glucosidase breaks the bonds between the two glucose sugars of cellobiose to produce monomers of glucose (FIG. 2). This synergistic effect of these enzymes makes possible the cellulose hydrolysis to glucose (Wood T M. Synergism between enzyme components of Penicillium pinophilum cellulase in solubilizing hydrogen bond ordered cellulose. J Biochem 1989; 260:37-43; Wood T M, McRae S T. The cellulase of Trichoderma koningii: purification and properties of some endoglucanase components with special reference to their action on cellulose when acting alone and in synergism with the cellobiohydrolase. J Biochem 1978; 171:61-9; Wood T M, McRae S T. Synergism between enzyme involved in the solubilization of the native cellulose. Adv Chem Ser 1979; 18:181-210). That is, at least three types of enzymes e.g. endoglucanases, cellobiohydrolases and β-glucosidase and their synergistic effect are required for complete digestion of cellulose. For each of these three enzymes different structural variants exist that perform the same function. Many more enzymes are required to digest hemicelluloses to monomer sugar including xylanase, xylosidase, arabinofuranosidase, mannanase, galactosidase and glucuronidase.
It is an object of the present invention to provide isolated polypeptides having cellulolytic activity and isolated nucleic acid sequences encoding the polypeptides to improve the conversion of cellulosic materials into fermentable sugar.